Both commercial and miliary aircraft typically carry an auxiliary power unit (APU) and often additionally may utilize a so-called emergency power unit (EPU). In some instances, the functions of both are combined.
In emergency systems, EPU's or APU's that operate additionally as EPU's must be brought into full operational capacity in a relatively short period of time, such as two or three seconds. In the usual case, these systems employ a turbine wheel for driving emergency power sources such as an electrical generator, hydraulic pump or both so as to provide the energy necessary to continue to operate the aircraft. Consequently, it is necessary that the turbine wheel be accelerated up to normal operating speed in a relatively short period of time so that if an APU is being utilized to provide emergency power, it can reach a self sustaining speed. Where an EPU is being utilized, it still must be accelerated rapidly and then its operation maintained for some predetermined time period.
Typically, these systems include a storage source for fuel and a high pressure storage vessel for oxidant which is utilized to combust the fuel. The oxidant may be air, oxygen enriched air, or even molecular oxygen.
Because of volume and weight constraints typically associated with aircraft, it is desirable to make the storage vessels as small and as lightweight as possible and that in turn means that it is desirable to hold oxidant requirements for a given emergency operation to a minimum. One way, of course, to minimize oxidant consumption, and thus the need for oxygen storage volume, is to control the flow of oxidant to a combustor where it is employed to combust fuel to provide motive gases for the turbine wheel, so as to provide only the amount of oxidant required to effect the desired combustion. Consequently, in an EPU, for example, it will be desirable to sense the power demand of the aircraft which is being placed on the turbine wheel of the EPU and regulate the flow of both fuel and oxidant appropriately.
Unfortunately, proper regulation of the flow of oxidant is not always easily achieved. Frequently, at extremely high altitudes, the oxidant storage vessel and flow control valves will be at extremely low temperatures that make operation of the valves and other flow regulating components difficult. The problem is exacerbated by the Joule-Thompson effect when an oxidant storage vessel is opened to initiate the flow of oxidant to the combustor. The Joule-Thompson effect causes an instantaneous drop in temperature of a gas as a result of the adiabatic expansion of that gas. Thus, when a valve controlling the outlet of a storage vessel in a stored energy system opens, the oxidant expands as it flows from the vessel. Consequently, in a bottle where an oxidant is stored at high pressure, say 5500 psi, the storage vessel may be at -40.degree. F. due to ambient conditions. Opening of the storage vessel will cause an instantaneous decrease in temperature to -90.degree. F. and at the end of the operational procedure, gas temperatures as low as minus 190.degree. F. have actually been recorded. These conditions can make flow control unreliable.
The present invention is directed to solving one or more of the above problems.